Magnetic resonance imaging using contrast agents bioactivated by enzymatic cleavage

Abstract

The present invention relates to contrast agents for diagnostic magnetic resonance imaging. In particular, this invention relates to novel compounds which exhibit surprisingly improved relaxivity due to improved binding of an amino acid targeting group within the molecules to proteins following specific cleavage of the agent by a peptidase. This invention also relates to pharmaceutical compositions comprising these compounds and to methods of using the compounds and compositions for contrast enhancement during magnetic resonance imaging.

We claim:  
     
         1 . A magnetic resonance imaging contrast agent having the structure:  
       
         
           
           
               
               
           
         
       
       wherein: 
 the chelating ligand forms a complex with one or more paramagnetic metal ions selected from the group consisting of metal ions with atomic numbers 13, 21-34, 39-42, 44-50, and 57-83;  
 the linker, if present, comprises from 1 to 20 carbon atoms wherein the carbon atoms form a linear, branched, or cyclic alkyl group, aryl groups, or heterosubstituted analogs thereof wherein from 1 to 6 carbon atoms are replaced by nitrogen, oxygen or sulfur atoms;  
 n is 0 or 1;  
 the targeting amino acid comprises from one to three phenyl rings attached as a side chain at the α carbon of the amino acid wherein: 
 each phenyl group is optionally substituted with up to five substitutents selected from the group Z; wherein 
 Z consists of halogen, CN, NO 2 , CF 3 , OCF 3 , OH, S(C 1 -C 4 )-alkyl, SO(C 1 -C 4 )-alkyl, SO 2 (C 1 -C 4 )-alkyl, NH 2 , NH(C 1 -C 4 )-alkyl, N((C 1 -C 4 )-alkyl) 2 , COOH, C(O)O(C 1 -C 4 )-alkyl, O(C 1 -C 4 )-alkyl; (C 1 -C 6 )-alkyl, (C 2 -C 6 )-alkenyl, (C 2 -C 6 )-alkynyl, and (C 3 -C 7 )-cycloalkyl;  
 
 
 the masking polypeptide comprises from one to ten amino acid residues residues; and wherein 
 a cleavage site exists between the targeting amino acid and the masking polypeptide and said cleavage site is a peptide bond.  
 
 
     
     
         2 . The compound according to  claim 1  wherein the chelating ligand is selected from tetraamine 1,4,7,10-tetraazacyclododecane-N,N′,N″,N″-tetraacetic acid (DOTA), 1,4,7,10- tetraazacyclododecane-1,4,7-triacetic acid (DO3A), or one of the following structures:  
       
         
           
           
               
               
           
         
       
       wherein: 
 R is selected from —CX 2 C(O)O— or —CX 2 C(O)NX 2 ;  
 X is H, (C 1 -C 6 ) straight or branched chain alkyl, (C 2 -C 6 ) straight or branched chain alkenyl, or (C 2 -C 6 ) straight or branched chain alkynyl;  
 the single bond with the intersecting wavy line is the point of attachment for the linker; and  
 C(O) indicates a carbon-oxygen double bond.  
 
     
     
         3 . The compound according to  claim 2  wherein Z is halogen, CN, NO 2 , CF 3 , OCF 3 , or OH.  
     
     
         4 . The compound according to  claim 3  wherein the targeting amino acid is 3,5 diiodotyrosine.  
     
     
         5 . The compound according to  claim 3  wherein said targeting amino acid is a diphenylalanine group.  
     
     
         6 . The compound according to  claim 3  wherein the paramagnetic metal ion is selected from the group of metals having atomic numbers 21-29, 42, 44 or 57-83.  
     
     
         7 . The compound according to  claim 6 , wherein the paramagnetic metal ion is Gd 3+ .  
     
     
         8 . The compound according to  claim 7  wherein the chelating ligand and paramagnetic metal ion is gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA).  
     
     
         9 . The compound according to  claim 3 , wherein the metal chelate has a formation constant of greater than about 10 20  M −1 .  
     
     
         10 . The compound according to any of  claims 1  to  9 , wherein the covalently attached, cleavable masking polypeptide group is removed by an enzyme selected from the group consisting of Thrombin Activatable Fibrinolysis Inhibitor (TAFI), a member of the Carboxypeptidase B family, trypsin, Factor Xa, 7B2 protein, proprotein convertase 2, subtilisin, kexin endoproteinase, pancreatic carboxypeptidase, Endoproteinase Lys-C, Myxobacter Protease, elastase, matrix metalloproteinases (MMPs), Clostripain, and Armillaria Protease.  
     
     
         11 . The compound according to  claim 10 , wherein the covalently attached, cleavable masking polypeptide group is removed by TAFI or another member of the Carboxypeptidase B family.  
     
     
         12 . A pharmaceutical composition comprising a contrast agent according to any one of  claims 1  to  9 , and a carrier, adjuvant or vehicle.  
     
     
         13 . A method of MRI imaging comprising the step of administering a diagnostically effective amount of a compound according to any one of  claims 1  to  9 .  
     
     
         14 . A method for MRI imaging comprising the step of administering a diagnostically effective amount of a composition according to  claim 12 .  
     
     
         15 . A method of bioactivating a contrast agent comprising the steps of administering a compound according to any one of  claims 1  to  9 , and activating the contrast agent via cleavage of the masking polypeptide by a protease.  
     
     
         16 . A method for bioactivating a contrast agent comprising the steps of administering a pharmaceutical composition according to  claim 12 , and activating the contrast agent via cleavage of the masking polypeptide by a protease.